Intrusion detection systems for detecting and signaling an alarm if a person or persons intrude into a surveillance area are well known. Optical, infrared, seismic, sonic, radar, and like sensing techniques are employed for such purpose. For outdoor, all-weather surveillance, continuous wave (CW) radar often is used since it is much less susceptible to inclement weather conditions such as heavy rain, fog, and snow than the other techniques. Examples of CW radar intrusion detection systems are to be found in U.S. Pat. Nos. 3,242,486--Corbell; 3,859,656--Klein et al; and 3,803,599--McLean et al. A major operational deficiency of many prior art CW radar counterintrusion systems is that false alarms are frequently caused by small animals that meander into the surveillance area and by very large moving objects, such as trucks, that move well outside the surveillance area. The principal reason for this deficiency is that currently available radar sensors which are suitable for unattended alarm system applications cannot distinguish among any of the following:
1. Small objects that are moving in close proximity to the sensor, inside the surveillance area; PA0 2. Large objects moving a relatively long distance well outside the boundary of the surveillance area; and PA0 3. A person entering the threshold of the surveillance area.
This inability to distinguish objects often occurs because the sensing mechanism is sensitive to and operates on the amplitude of the radar echo received from a moving object, which can be the same amplitude for a very small object at short distances or a very large object at a long distance.
A prior art solution to part of this problem of false alarms is disclosed in U.S. Pat. No. 3,815,131--Dautel et al which discloses a two-sensor system in which the surveillance area is located between the two sensors which are mounted facing each other. To avoid interference, the sensors operate at different frequencies. Also, the antennas are designed so as to suppress side and back lobes. Outputs from the sensors are combined in an AND gate whereby only when a target is detected by both sensors will an alarm be initiated. Thus, if a small animal or bird comes close to one of the sensors it will be detected only by that particular sensor since it is too far from the opposite sensor, and no alarm will be initiated. Similarly, a large vehicle moving some distance away in the field of one sensor may be detected by that sensor but is less likely to be detected by the opposite sensor since it is in the suppressed side- or back-lobes thereof. Hence, no alarm is initiated. A major disadvantage of such an arrangement is that there remain significant areas in the detection geometry where energy reflected from large reflecting objects will be detected by both sensors causing false alarms. Examples are: (a) where a chain-link security fence is running parallel to the axis of the sensors and the fence is vibrated by the wind and (b) when large vehicles, such as trucks, move parallel to the sensing axis.